1. Field of Invention
The present invention relates to a method for preparing silicon and boron oxynitrides. The present invention also relates to the silicon and boron oxynitrides obtained and the polyborosiloxanes obtained as intermediates in such a process.
2. Description of Related Art
A number of compounds are known that are refractory ceramics, for example silicon Si.sub.3 N.sub.4 and silicon oxynitride. Such materials are useful for their mechanical properties at high temperatures, particularly their resistance to heat impact, to corrosion, and to oxidation. In addition, boron nitride has a low density, a high melting point, a high thermal conductivity, and chemical inertia.
These materials can be obtained in various forms, particularly in the form of fibers or films, by the organometallic method for example. However, the mechanical properties of silicon nitride fibers degrade when they are (1) subjected to a temperature higher than 1400.degree. C. due to crystallization of Si.sub.3 N.sub.4, or (2) placed in an oxidizing environment which brings about chemical transformation.
Materials of the SiBON type have been proposed. These materials combined good thermomechanical properties with exceptional oxidation resistance at high temperatures. However, these materials are either precursors of materials in which Si.sub.3 N.sub.4 is the essential component and have the aforementioned drawbacks or they are obtained from precursors of the insoluble and/or nonmelting polyborosiloxane type as described, for example, by L. Venisse et al., High Temp. Chem. Processes 1 (1992) 129-139 and High Temp. Chem. Processes 1 (1992) 229-239, the disclosures of which are incorporated herein by reference. The insoluble and/or nonmelting nature of the precursors makes it difficult or even impossible to prepare boron and silicon oxynitrides in fiber form.
A known method of preparing a boron and silicon oxynitride consists of preparing a poly(borosiloxane) precursor; conducting nitriding pyrolysis of the precursor to obtain an amorphous boron and silicon oxynitride; and optionally conducting additional pyrolysis to obtain a crystallized boron and silicon oxynitride.
It is known to prepare a poly(borosiloxane) precursor by polycondensation of a boron compound comprising one or more OH groups attached to the boron and a poly(hydrogenosiloxane) that has at least one Si--H bond per monomer unit in the presence of a catalyst.
The goal of the present invention is to furnish polyborosiloxanes usable for preparing materials of the SiBON type that, in addition to the excellent thermomechanical properties of known ceramics, exhibit improved resistance to oxidation with soluble and/or melting intermediates.